The present invention generally relates to data transmission methods and data transmission devices, and, more particularly, a SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical Network) transmission system that complies with the Ethernet (a registered trade name) protection system, and increases a transmission efficiency by applying the Ethernet protection system to the SDH/SONET transmission system.
In recent years, as Ethernet LANs have been widely used, there is an increasing demand for suitable protection provided to prevent packet transmission data from being damaged even when a fault occurs in a transmission path. In response to such a demand, there is a method called “dual-homing” method.
FIGS. 1A and 1B illustrate a protection system in accordance with a dual-homing method. As shown in FIG. 1A, this system includes a master LAN switch LS-M and a slave LAN switch LS-S, and a server 10 is connected to clients 21 and 22 via working paths P-W1 and P-W2 and back-up paths P-P1 and P-P2. In this system, actual data packets are normally transmitted through the working paths P-W1 and P-W2, while control packets are transmitted through the back-up paths P-P1 and P-P2.
It is assumed here that a fault has occurred in the master LAN switch LS-M, to which the working paths P-W1 and P-W2 are connected, and, as a result, it has become impossible to transmit actual data packets through the working paths P-W1 and P-W2 in the above structure. In such a case, the fault is detected by the provided protection system, and switching control is then performed on the master LAN switch LS-M and the slave LAN switch LS-S, so that the actual data packets can be transmitted through the back-up paths P-P1 and P-P2, through which the control packets have been transmitted until then, instead of the working paths P-W1 and PW2. The switching control is performed at a high speed so as to switch the paths without causing a time lag. By doing so, an accident such as inadvertent discard of data packets can be prevented.
In the above dual-homing system, the backup paths P-P1 and P-P2 need to take all data traffic, instead of the working paths P-W1 and P-W2, but, in a normal operation, the back-up paths P-P1 and P-P2 transmit only the control packets. This implies that only a half of the capacity of the four paths P-W1, P-W2, P-P1, and P-P2, is utilized, which presents a problem of poorer path usability.
To use the LANs of the above Ethernet type over a long distance, a communication system using a multiplexing communication network SONET has been developed.
FIGS. 2A and 2B illustrate a structure in which LANs of the Ethernet type are applied to (or mapped onto) a SONET so as to realize LANs over a long distance.
In this case, the above four paths P-W1, P-W2, P-P1, and P-P2 need to be realized in the SONET, and, to do so, the corresponding transmission bandwidth needs to be ensured on the optical cable transmission paths that constitute the SONET. Furthermore, it is a known fact that a SONET has its own protection system (or redundant structure), and only a half of the transmission bandwidth of the SONET is utilized accordingly. When LANs of the dual-homing Ethernet type provided with the above protection are applied to a SONET, the resultant usability of the transmission bandwidth will be only a fourth of the usability of a structure that is not provided with a protection (or a redundant structure).